1. Field of the Invention
The present invention generally relates to a re-transmission control method and a relay station apparatus in a relay communication system, and more particularly to a re-transmission control method and a relay station apparatus in a relay communication system used for mobile communications.
2. Description of the Related Art
In recent years and continuing, there is an increasing use of mobile communications for the purpose of data communications with, for example, portable telephones and wireless LAN. Thus, wireless broadband access systems for achieving faster data communications are being studied.
Compared to conventional communication systems used for purposes such as telephone communications, wireless broadband access systems are expected to provide higher wireless quality. Furthermore, since transmission power of mobile stations is limited due to constraints such as their limited power capacity, it is difficult to expand the wireless area that can be covered by a single wireless base station. Under these circumstances, in order to expand communication service areas and increase communication quality, studies are being made for relay communication systems performing relay communications by positioning relay stations at outer end parts of communication service areas or dead areas where radiowaves cannot reach and performing communications between wireless base stations and mobile stations via the relay stations.
In mobile communications, transmission paths change along with the passage of time due to, for example, movement of the mobile station or changes in the radio-wave environment (surrounding mobile objects, weather, etc.). Therefore, each data block (packet) of communication data has added error correction/detection symbols for allowing a receiver station to determine whether there are any errors in the decoded or reproduced data. In a case where an error is detected, the receiver station requests for the communication data to be re-transmitted. One commonly known method for automatically performing a re-transmission process is an ARQ (Automatic Repeat Request).
Furthermore, another well known method for automatically performing a re-transmission process is a HARQ (Hybrid Automatic Repeat Request) method which effectively uses received data that have not been successfully received (error-detected data). With the HARQ method, a receiver station buffers error detected data and subsequently combines the buffered data with re-transmitted data, to thereby perform a decoding process on the combined data.
In a case where such re-transmission methods are used in the above-described relay communication system, it is necessary to optimize re-transmission control by considering, for example, transmission quality of plural transmission paths between a transmitting station and a receiving station via one or more relay stations and process delay of each relay station and receiving station.
Japanese Laid-Open Patent Application No. 2001-196990 (hereinafter referred to as “Patent Document 1”) discloses a methodology of performing re-transmission control in a relay transmission system by positioning a relay transmission apparatus between a data transmitting apparatus and a data receiving apparatus and relaying data without simultaneously performing data transmission and data reception.
FIG. 14 shows an example of a communication system (to which the present invention may be applied). As shown in FIG. 14, communications are performed between a wireless base station (BS) and mobile stations (MS) #0-#2 via a relay station (RS). It is to be noted that communications may also be performed directly between the wireless base station (BS) and a mobile station (MS) #3 without the intervention of the relay station (RS).
The transmission path between the wireless base station (BS) and the relay station (RS) is hereinafter also referred to as a “relay link” and the transmission path between the relay station (RS) and the mobile station (MS) is hereinafter also referred to as an “access link”. In a case of performing communications between the wireless base station (BS) and the mobile station (MS) where there are two types of wireless link paths (relay link and access link), communication data error may occur in either one of the links according to, for example, the statuses of the transmission paths.
FIG. 15 is a schematic diagram showing an exemplary configuration of a wireless frame for relay communications. In the example shown in FIG. 15, one frame period is divided into four time phases. The relay link is assigned to a communication period between the wireless base station (BS) and the relay station (RS). The access link is assigned to a communication period between the relay station (RS) and the mobile station (MS).
Access timings for both communication directions are assigned to the relay link and the access link, respectively. The communication paths assigned to the access timing for communication from the wireless base station (BS) and the relay station (RS) and communication from the relay station (RS) and the mobile station (MS) are hereinafter also referred to as “downstream link”. The communication path assigned to the access timing for communication from the relay station (RS) to the wireless base station (BS) and communication from the mobile station (MS) to the relay station (RS) are hereinafter also referred to as “upstream link”.
More specifically, the communication path from the wireless base station (BS) to the relay station (RS) is referred to as “downstream relay link”, the communication path from the relay station (RS) to the mobile station (MS) is referred to as “downstream access link”, the communication path from the mobile station (MS) to the relay station (RS) is referred to as “upstream access link”, and the communication path from the relay station (RS) to the wireless base station (BS) is referred to as “upstream relay link”.
Plural data blocks of communication data or plural response signals can be transmitted within the communication periods of each link by using, for example, time division multiplexing, frequency division multiplexing, or code division multiplexing. Furthermore, data regarding the number of transmission data blocks (transmission data number) or frame format are transmitted as control data from the wireless base station (BS) in the downstream link.
The example shown in FIG. 15 is a frame in a case where the wireless base station (BS) transmits plural data blocks (DATA #1-#4) bound for the mobile station (MS) in the downstream link and the mobile station (MS) transmits response signals corresponding to the received data blocks to the wireless base station (BS) in the upstream link. In this case, the mobile station (MS) detects errors in data blocks DATA#2 and DATA#4 and transmits negative response signals (NACK) #2 and #4 requesting re-transmission with respect to DATA#2 and DATA#4 in the upstream link. In addition, the mobile station (MS) transmits affirmative response signals (ACK) #1 and #3 indicating successful receipt of data with respect to DATA #1 and #3.
It is to be noted that the exemplary frame configuration of FIG. 15 is illustrated without considering delay of data blocks or response signals (e.g., a case where delay of a data symbol decoding process or an error detection process by the mobile station (MS) causes response signals to reach the wireless base station a few frames after and prevents the reception signals from reaching the wireless base station (BS) within the same frame as the transmission of its corresponding data blocks, and a case where delay of a relay process by the relay station (RS) causes data blocks or response signals to be relayed one or more frames after the frame of receiving the data blocks or the response signals).
FIG. 16 is a sequence diagram showing data communication according to the frame shown in FIG. 15. As shown in FIG. 16, the wireless base station (BS), after receiving a data transmission request (16-1), generates control data including transmission data number or frame format (16-2) and transmission data blocks (16-3). Then, the wireless base station (BS) transmits the generated control data (in this example, control data #1) and data blocks (in this example, DATA #1, #2, #3, and #4) to the mobile station (MS) via the relay station (RS).
The mobile station (MS) receiving the control data #1 extracts a control data item(s) included in the control data #1 (16-4). Furthermore, the mobile station (MS) receiving the DATA #1, #2, #3, and #4 performs error detection on the DATA #1, #2, #3, and #4 (16-5). Then, the mobile station (MS) generates response signals indicating the results of the error detection (16-6). Then, the mobile station (MS) transmits response signals ACK #1, ACK #3, NACK #2 and NACK #4 to the wireless base station (BS) via the relay station (RS).
Then, the wireless base station (BS) receiving the response signals ACK #1, ACK #3, NACK #2 and NACK #4 from the mobile station (MS) detects a re-transmission request (16-7) and generates re-transmission control data (16-8). Then, the wireless base station (BS) transmits the generated re-transmission control data as control data #2 to the mobile station (MS) via the relay station (RS). Furthermore, the wireless base station (BS) generates re-transmission data blocks (16-9) and transmits the generated re-transmission data blocks as re-transmission data blocks DATA #2 and DATA #4 to the mobile station (MS) via the relay station (RS).
The mobile station (MS) receiving the re-transmission control data #2 extracts a control data item(s) included in the control data #2 (16-10). Furthermore, the mobile station (MS) receiving the re-transmission data blocks DATA #2 and DATA #4 performs error detection on the re-transmission data blocks DATA #2 and DATA #4 (16-11). Then, the mobile station (MS) generates response signals indicating the results of the error detection (16-12) and transmits the generated response signals ACK #2 and ACK #4 to the wireless base station (BS) via the relay station (RS).
In the relay station (RS) according to the above-described sequence diagram, control data and data blocks received through the relay link from the wireless base station (BS) are directly transmitted as they are to the access link. Furthermore, response signals received through the access link from the mobile station (MS) are directly transmitted as they are to the relay link.
Accordingly, since the relay station (RS), receiving data blocks through the downstream relay link from the wireless base station (BS), transfers all the received data blocks to the mobile station (MS) through the downstream access link without performing error detection on the received data blocks, invalid data cannot be prevented from being transmitted from the relay station (RS) in a case where there is an error in the data blocks received from the wireless base station (BS) since the data blocks are transmitted as they are to the downstream access link.
Accordingly, such transmission by the relay station (RS) is a waste of transmission power and is an ineffective use of wireless transmission resources (Problem 1). Furthermore, in such a case, processes of the mobile station (MS) become delayed since the decoding process and the error detection process are performed based on the invalid data. Using power for performing such processes is also a waste of power (Problem 2).
FIG. 17 is a schematic diagram showing an exemplary frame configuration in a case where error data are not transmitted to a downstream access link when a relay station (RS) detects an error in the data in a downstream relay link. Furthermore, FIG. 18 is a sequence diagram showing data communication according to the frame shown in FIG. 17. In the sequence diagram shown in FIG. 18, the relay station (RS) performs error detection on data blocks DATA #1, #2, #3, and #4 received through the downstream relay link (18-1), deletes DATA #4 from which an error is detected (18-2), and transfers errorless data blocks DATA #1, #2, and #3 to the downstream access link. Thereby, an empty space is created in the downstream access link since the data block #4 is not transferred to the downstream access link.
Then, in the same manner as FIG. 16, the mobile station (MS), receiving control data #1 from the base station (BS) via the relay station (RS), extracts a control data item(s) from the control data #1 (16-4). Then, the mobile station (MS), receiving the data blocks Data #1, #2, and #3 from the base station (BS) via the relay station (RS), performs error detection (including detection of unreceived data) on the data blocks DATA #1, #2, and #3 (16-5) and generates a response signal (16-6). In a case where the mobile station (MS) detects that an error in data block Data #2 and no reception of data block DATA #4, the mobile station (MS) returns (transmits) negative response signals NACK #2 and #4 to the wireless base station (BS) via an upstream access link and an upstream relay link.
Then, the relay station (RS) also performs error detection on the re-transmission data blocks DATA #2 and #4 (18-3), deletes an error-detected re-transmission data block(s) if any errors are detected in the re-transmission block (18-4), and transmits an errorless data block DATA #2 and #4 to the mobile station (MS).
As shown in FIGS. 17 and 18, although the above-described Problem 1 may be overcome by deleting error-detected data blocks and preventing the error-detected data blocks from being transferred to the mobile station (MS) at the relay station (RS), the above-described Problem 2 cannot be resolved. Furthermore, available (empty) wireless transmission resources, which are created by preventing the error-detected data blocks from being transferred to the mobile station (MS), cannot be effectively utilized (unless some modifications are made) (Problem 3).
That is, in general, the mobile station (MS) transmits and receives data blocks based on control data (designating, for example, transmission timing, modulation method, and error correction coding method) from the wireless base station (BS). Therefore, even where the relay station (RS) deletes error data blocks and creates an empty (available) space in the downstream access link, the available wireless transmission resource cannot be utilized unless changes are made in the designation content of the control data.
Although the relay transmission system disclosed in Patent Document 1 performs an operation similar to that described with FIGS. 17 and 18, the relay station in the relay transmission system is provided with a re-transmission buffer for storing data transmitted from a transmitting station therein. Accordingly, in a case where the relay station receives a re-transmission request from a receiving station, the relay station, without transferring the re-transmission request to the transmitting station, transmits the data stored in the re-transmission buffer as re-transmission data to the receiving station.
Although the relay transmission system disclosed in Patent Document 1 can omit the process of transmitting a re-transmission request to an upstream relay link, the relay transmission system requires a re-transmission buffer to be provided in the relay station. This complicates the storing of data in the re-transmission buffer and managing the disposal of the data stored in the re-transmission buffer (Problem 4). Furthermore, this relay transmission system has the same problem as Problem 3 in which there is no method of effectively utilizing wireless transmission resources of the downstream access link that are created when error-detected data blocks are prevented from being transferred.
FIG. 19 is a schematic diagram showing an exemplary frame configuration in a case where a relay station buffers signals received from the downstream relay link and the upstream access link and separately operates the relay link and the access link asynchronously. FIG. 20 is a sequence diagram showing data communication according to the frame shown in FIG. 19. With the configuration shown in FIGS. 19 and 20, the relay link and the access link can effectively use the available wireless transmission resources separately. However, this configuration requires a re-transmission buffer to be provided in the relay station (RS) in correspondence with the downstream link and the upstream link, respectively. This increases the size of the relay station (RS) and causes difficulty in mounting the relay station (RS).
Furthermore, since the access link is operated asynchronously with the relay link, the wireless base station (BS) is unable to have a direct grasp (comprehension) of the status of communication quality of the mobile station (MS). Therefore, in a case where, for example, the wireless base station (BS) provides a priority class (QoS) with respect to each mobile station in the entire service area, the wireless base station (BS) does not have uniform control over its wireless resources during scheduling and assigning the wireless resources according to priority class (QoS) This makes it difficult for the wireless base station (BS) to assign wireless resources in correspondence with communication quality of the mobile station (MS) (Problem 5).